Conventional utility poles for electricity power transmission lines, street lamps and the like are formed from steel, concrete or wood.
Wooden poles have the advantages that they are natural, of very low cost for relatively short poles and are readily drillable for attachment of various mounting brackets and the like. In addition wooden poles effectively dampen vibration of the pole to resist galloping of the wires caused by vibration at a natural frequency. The wood poles are however relatively weak for the weight of material and have a significant problem with deterioration. The use of preservatives can prevent deterioration but leads to environmental problems. While wooden poles are very hard to displace at short lengths for the significant cost advantages, longer lengths of pole become very expensive due to the unavailability of trees of the required dimensions.
Concrete poles are very strong and relatively flexible but have the significant disadvantage of being very heavy, brittle and difficult to drill. The total cost of an installed pole is also dependent upon the cost of transportation and on site installation of the pole and concrete in view of its great weight significantly increases the costs in these areas.
Steel poles are very strong and flexible but have the significant disadvantages of rusting, high cost and being electrically conductive. This leads to increased costs in relation to the insulators necessary to seperate the power cables from the electrically conductive metal pole.
It has been known for many years, therefore, that there is a significant advantage in utilizing fiber reinforced plastics material for the manufacture of poles of this type. Many attempts have been made to develop poles manufactured from these materials but up till now the only practical pole construction utilized commercially is that manufactured by Shakespeare of Newbury South Carolina which provides a pole for supporting lighting and electrical power cables where the transverse or bending forces on the pole can be significantly increased.
The Shakespeare pole is manufactured by filament winding techniques in which fibers impregnated with resin are wrapped helically around a central mandrel to form a wall thickness of the required amount. The central mandrel is then removed after curing of the resin to leave the finished pole which can then be coated by various techniques to provide an attractive and resistant outer surface.
Various other techniques have been proposed for manufacture of poles for supporting lighting, electric power cables and the like. For proper reinforcement of the pole structure, it is known that both longitudinal and transverse fibers are required. A number of techniques are proposed for providing such arrangement of fibers. Pultrusion is a well known technique in which parts are formed of a constant cross section by drawing through a dye resin impregnated fibrous materials which are continuous along the length of the structure. The majority of the fibers in such a structure are longitudinal but some transverse fibers can be added by the addition of various types of mat which has the combination of longitudinal and transverse fibers. One example of a pole of this type is shown in U.S. Pat. No. 4,803,819 (Kelsey) and described in a paper dated 1987 relating to a product designed by 3-P Industries Inc. of Toms River N.J. This arrangement provides a pole of constant transverse cross section which is an essential result of the pultrusion process. In order to provide the required structural strength for this pole, a complex cross section is necessary including transverse webs both internally and externally of a main longitudinal cylindrical shape of the pole. It is understood that this arrangement has not achieved significant commercial success. The constant cross section of pultruded poles of this type is unsatisfactory since it does not maximize the strength to weight characteristics and since the constant cross section thus formed is significantly less resistant to damaging vibrations than are tapered poles. In addition the limited amount of transverse fibers that can be included significantly reduces the resistance of the pole to transverse collapse thus requiring the complex transverse elements of the cross section which are difficult and expensive to manufacture.
Filament winding provides another technique for providing both the longitudinal and transverse fibers. In this technique the fibers are wound helically about the central axis of the pole. It is of course possible to control the helix angle so that at some point along the length of the pole the helix angle is very shallow that is close to 90.degree. to the axis. At other points along the length of the pole, the helix can be stretched out to approximate to longitudinal fibers. However it is certainly not possible to apply such fibers directly longitudinally since it is necessary to maintain some degree of helix angle to hold the fibers in place on the structure. It will be appreciated that complex computer control of the winding can be effected both in relation to the helix angle and the reversal points of the helix to maximize structural strength. In addition the shape and wall thickness of the pole can be varied again to maximize strength both in relation to the taper of the pole and the thickness of the wall of the pole at various positions along the length of the pole. This arrangement is shown in a catalogue of Shakespeare which shows the various products available. A further example is shown in a paper published in October 1980 which refers to a pole of this type manufactured by W.J. Whatley Inc. of Commerce City Colo.
Up till now the manufacture of poles from composite materials, apart from the limited success of the Shakespeare pole, has not lead to commercial acceptance. The relatively high cost of the composite material related to the availability, low cost and acceptance of the wooden pole has prevented the composite pole from replacing the wooden pole.
Some attention has therefore been given to manufacturing extension pieces which are mounted on the top of the pole. The extension pieces are conventionally manufactured from composite materials using various techniques and are attached to the top of the wooden pole. However, this technique does not overcome the environmental problem of wooden poles. In addition, the mounting of an extension piece at the top of the pole is limited in that the basic strength of the pole is determined by its thickness and density of its base so that a pole having a pre-determined thickness at the base cannot be extended beyond a pre-determined limit without exceeding the strength of the base and thus overloading the pole beyond its capabilities.
In addition, wooden poles have a tendency to rot at the base, particularly in ground conditions of heavy moisture. Some attempts therefore have been made to provide additional support at the base by driving stakes into the ground around the base of the pole and attaching the pole to the stakes.
A further technique involves a provision of concrete sleeves which are embedded in the ground and the pole carried in the sleeve supported by aggregate located between the sleeve and the pole.
However, all of these procedures have limitations and have not been able to solve the basic environmental problems of wooden poles